"Drill steels" is the term applied to hollow tubes which are used to transmit rotating power from a power source to a drill bit. The steels telescope together to provide a drill train as long as desired for a particular application. The telescopic connection is usually achieved by an ensmalled portion or male end at one end of a steel which fits into a recess at the other end of a corresponding drill steel. The so-called male end or insertion end has been applied to the drill steel by placing and joining an insert part way into the drill steel tube. The insert is joined to the exterior of the drill steel by plug welds 180.degree. apart made in holes or slots premachined in the drill steel. Thus, a portion is extending from the tube and insertable into an adjoining tube.
The present invention contemplates a drill steel which is formed in a two-step process which eliminates the need for any welding and insures a tighter connection for the male end. In addition, the elimination of welds avoids the creation of as-cast and heat-affected zones in the microstructure of the weldments, and also reduces the possibilities for detrimental residual stresses and notchtype defects often associated with weldments.
The improved method of joint assembly not only eliminates the welds, but it provides a stronger joint with true, concentric alignment of the insert in the drill steel and thus improving the straightness of the entire drill assembly. The cost of manufacturing is reduced by the time-saving two-step process.
The preferred method for fabrication of the joint assembly of this invention is by hot upset forging, although other methods such as open or closed die forging and back extrusion might be utilized.
Objects and advantages will be apparent in the following description and claims in which the invention is described in detail to enable a person skilled in the art to practice it, all in connection with the best mode presently contemplated for the invention.